Electro-optical techniques have been used in detecting objects or targets. For example, an imaging system can be fitted on an aircraft that flies by an area containing an object. The imaging system may optically capture images of the area, which may be subsequently analyzed to determine the existence and location of the object. Typically, this analysis is slow and labor-intensive as it requires the images to be searched by a combination of machine and human image analysts. Additionally, the images may not capture the object in some unfavorable environmental conditions such as clouds, rain, snow, or at night time.
Imaging using radio frequency (RF) energy has also been traditionally employed. For example, the imaging system may employ synthetic aperture radar (SAR) and inverse SAR techniques to generate geospatial maps or images from RF energy. SAR is a form of radar that uses the relative motion between an antenna of the imaging system and the object to provide distinctive long-term coherent-signal variations that are exploited to obtain finer spatial resolution than is possible with conventional beam-scanning systems. The waveforms received successively at the different antenna positions are coherently detected, stored, and processed together to detect the object in an image of the area.
The use of such SAR techniques may be limited by the structure or dimensions of the object. For example, the signature of a large non-radiating object, such as a very high frequency (VHF) passive antenna array, may have a minimal amplitude deviation as compared to other elements in the area and may render the object almost invisible in a SAR map.